We add the endings -er and -est to words when we want to compare things.
If the word has a short vowel sound just before the last letter we double the last letter before we add the -er and -est.
big / bigger / biggest
If there are two consonants (words that are not vowels) after the short vowel sound we don't double the last letter.
long / longer / longest
If the vowel sound is long we don't double the last letter.
cheap / cheaper / cheapest
Finally, there are some words that don't obey the rules at all!
good / better / best
bad / worse / worst